A wind turbine generator, which transmits generated electrical power to a utility grid, is required to perform PFR (primary frequency control) for controlling the generated output power so as to stabilize the grid frequency, which is the frequency of the utility grid, in order to maintain the quality of electrical power and stabilize the utility grid. Thus, when the grid frequency is increased, the wind turbine generator needs to reduce active power to be output.
Table 1 shows example requirements (interconnection requirements (grid code)) of the wind turbine generator in response to a change in grid frequency.
TABLE 1GRIDFREQUENCYREQUIREMENTS>47.5HzDISCONNECTION≧47.5HzNO LIMIT, CONTINUE NORMAL OPERATION<50.2Hz≧50.2HzLIMIT GENERATED OUTPUT POWER ACCORDING<51.5HzTO FOLLOWING LIMITING EQUATION  ΔP  =      20    ⁢          P      M        ⁢                  50.2        -                  f          grid                    50.0      PM: ACTIVE POWER THAT CAN BE CURRENTLYOUTPUT [MW]ΔP: AMOUNT OF LIMITATION ON GENERATEDOUTPUT POWER [MW]fgrid: GRID FREQUENCY [Hz]≧51.5HzDISCONNECTIONNotethat, as indicated by the limiting equation shown in Table 1, the amount of limitation on the generated output power is determined according to the difference between an actual grid frequency and a reference grid frequency (50.2 Hz in the example shown in Table 1).
If the wind turbine generator cannot satisfy the interconnection requirements, the wind turbine generator may not be allowed to interconnect to the utility grid.
PTL 1 describes a wind-turbine-generator control method for reducing the output power of a generator according to an increase in grid frequency. In the control method described in PTL 1, when the grid frequency is increased, the grid frequency is set as the control target, and the output power of a wind turbine generator is reduced so as to reduce the grid frequency.